1. Field
The invention relates to chiral dopants for liquid crystals and liquid crystalline mixtures which contain such dopants and their use for optical and electro-optical devices.
2. Background
Liquid crystal materials for cholesteric filters frequently contain, in addition to nematic liquid crystals, one or more optically active additives for the induction of a chiral structure. For example, for the use of such liquid crystals in optical filters a nematic liquid crystal is doped with optically active additives such that on the one hand a fixed rotational direction of the helix results and on the other hand the desired helical pitch is induced. In addition to the chiral properties of dopants, certain dielectric and optical properties are also desirable for optimal application in cholesteric filters. In order to achieve a bandwidth of the selective filter reflection which is as great as possible, chiral dopants having a large optical anisotropy are, for example, required. Further, cholesteric filters can be built with dopants of negative dielectric anisotropy which give rise to a planar orientation without disinclinations in electric fields. A high optical quality of the filter can thus be realized [M. Schadt, Liquid Crystals 14, 73 (1993)].
Cholesteric liquid crystals reflect selective light in a wavelength range for which the wavelength is about the same as the helical pitch. The spectral width of this reflection light can be varied by suitable choice of the liquid crystal. The reflected light is completely circularly polarized. The rotational direction of the reflected light depends on the direction of rotation of the cholesteric helical structure. The opposite circularly polarized light is transmitted unimpaired. These properties can be used for the production of optical filters, polarizers, analyzers etc.
Cholesteric liquid crystals for the above applications can preferably consist of a nematic or cholesteric basic material and one or more chiral dopants, which permits a simple adjustment of the desired helical pitch.
In order to achieve cholesteric mixtures having a pitch in the range of the wavelength of visible light, the chiral dopants should have a twisting capacity which is as high as possible and should have a good solubility in usual liquid crystal materials. Furthermore, the chiral dopants should have an adequate stability, should have a good compatibility with the mesophase type of the liquid crystal material and should not restrict the mesophase range too severely. Such properties would also be desirable for chiral dopants in order to produce the highly twisted nematic structures referred to earlier, as their amount can be kept low in order that the properties of the liquid crystal material are influenced only immaterially.